1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device and fabricating method thereof. Although the present invention is suitable for a wide scope of applications, it is particularly suitable for maximizing brightness and color reproducibility by improving a contrast ratio as well as preventing light leakage without reducing an aperture ratio.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are developed as next generation display devices because of their characteristics of light weight, thin profile, and low power consumption. Generally, an LCD device is a non-emissive display device that displays images using a refractive index difference having optical anisotropy properties of liquid crystal material that is interposed between a thin film transistor (TFT) array substrate and a color filter (C/F) substrate.
In the conventional LCD device, a displaying method using a backlight behind the array substrate as a light source is commonly used. However, the incident light from the backlight is attenuated during the transmission so that the actual transmittance is only about 7%. The backlight of the conventional LCD device requires high brightness, thereby increasing power consumption by the backlight device. Thus, a relatively heavy battery is required to supply a sufficient power to the backlight of such a device, and the battery cannot be used outdoors for a long period of time.
In order to overcome the problems described above, a reflective LCD has been developed. Since the reflective LCD device uses the ambient light instead of the backlight, it becomes light weight and easy to carry. In addition, power consumption of the reflective LCD device is reduced so that the reflective LCD device can be used for a portable display device such as an electronic diary or a personal digital assistant (PDA).
However, brightness of the reflective LCD device may vary in accordance with the surrounding conditions. For example, the brightness of the indoor ambient light differs largely from that of the outdoors. Therefore, the reflective LCD device cannot be used where the ambient light is weak or does not exist. In order to overcome such problems, a transflective LCD device has been researched and developed. The transflective LCD device can be switched from a transmissive mode using transmission of light to a reflective mode using reflection of light according to the user's selection.
FIG. 1 is a schematic cross-sectional view of a transflective liquid crystal display device according to a related art. As shown in FIG. 1, a liquid crystal panel 40 includes first and second substrates 10 and 30 facing into each other, and a liquid crystal layer 20 interposed therebetween. A transflective liquid crystal display (LCD) device 60 is composed of the liquid crystal panel 40 and a backlight unit 50. The backlight unit 50 is disposed at the outside of the liquid crystal panel 40 and provides the liquid crystal panel 40 with light.
A color filter layer 12 for passing the light having only the specific band of wavelength is formed on the inner surface of the first substrate 10. A common electrode 14 functioning as an electrode applying a voltage to the liquid crystal layer 20 is formed on the color filter layer 12. An insulating layer 32 is formed on the inner surface of the second substrate 30. A transparent pixel electrode 34 functioning as another electrode applying a voltage to the liquid crystal layer 20 is formed on the insulating layer 32. A passivation layer 36 and a reflecting layer 38 that commonly have a transmissive hole 35 exposing a portion of the pixel electrode 34 are subsequently formed on the pixel electrode 34. The liquid crystal panel 40 includes a reflective portion “r” corresponding to the reflecting layer 38 and a transmissive portion “t” corresponding to the transmissive hole 35.
In order to maximize the light efficiency of the reflective and transmissive portions “r” and “t”, a cell gap corresponding to a thickness of the liquid crystal layer 20 of the reflective portion “r” is designed to be different from that of the transmissive portion “t”. This structure is referred to as a dual cell gap structure. A cell gap “d1” of the transmissive portion “t” is about twice of a cell gap “d2” of the reflective portion “r.”
A retardation “δ” of a liquid crystal layer is defined by the following equation:δ=Δn·d,wherein δ represents a retardation of a liquid crystal layer, Δn is a refractive index anisotropy of a liquid crystal layer, and d represents a cell gap of a liquid crystal layer. Therefore, to reduce a difference in light efficiency between the reflective and transmissive modes, the retardation of the liquid crystal layer should be kept uniform by forming a cell gap of the transmissive portion larger than that of the reflective portion.
FIG. 2 is a schematic cross-sectional view of a transflective liquid crystal display device having a micro reflector structure (MRS) according to another related art. In FIG. 2, first and second substrates 70 and 90 face into and are spaced apart from each other, and a liquid crystal layer 65 is interposed between the first and second substrates 70 and 90. A transparent pixel electrode 92 is formed on the inner surface of the second substrate 90, and a passivation layer 96 having a transmissive hole 94 is formed on the pixel electrode 92. The transmissive hole 94 exposes a portion of the pixel electrode 92. The passivation layer 96 has an uneven pattern “A” on the upper surface. A reflecting layer 98 formed on the passivation layer 96 also has the transmissive hole 94 and the uneven pattern “A”. A color filter layer 72 and a common electrode 74 are subsequently formed on the inner surface of the first substrate 70. The reflective LCD device includes a reflective portion “rr” corresponding to the reflecting layer 98 and a transmissive portion “tt” corresponding to the transmissive hole 94.
Since the reflecting layer 98 has the uneven pattern “A” on the upper surface, the incident light is diffusely reflected at the reflecting layer 98 along several directions. Accordingly, the efficiency of the reflected light is improved. This structure of the reflecting layer is referred to as a micro reflector structure (MRS). The passivation layer 96 includes a plurality of seeds 96a having a hemispheric shape and a coating layer 96b covering the seeds 96a. In the MRS, even though the efficiency of reflected light is improved, it is difficult to control a step difference between the reflective and transmissive portions “rr” and “tt” in fabricating processes. This is due to severe variations in processing conditions for the coating layer 96b covering the seeds 96a in accordance with an area ratio of the transmissive portion “tt”. Moreover, in a transflective LCD device having a dual cell gap structure, light efficiency between reflective and transmissive portions is kept uniform. However, since a color filter layer has a uniform thickness at the reflective and transmissive portions, light passing through the reflective portion has a high-color reproducibility and a low-brightness as compared to light passing through the transmissive portion due to a difference between the numbers passing through color filters of the reflective and transmissive portions. Accordingly, a color difference between the reflective and transmissive portions occurs.